Proper delineation of floodways is crucial to a variety of rural management objectives, including the identification of flooding hazards and critical areas. However, the definition of floodways differ between various regulations, and may be determined differently based on stream types and the type of geospatial information used. For example, the Washington Shoreline Management Act defines the floodway based on biological criteria, which differs from the primarily physical description of a FEMA floodway. Shoreline managers are faced with the task of trying to determine consistent and defensible boundaries, using a variety of geospatial and field information including ordinary high watermarks, remotely sensed elevation data, geomorphic conditions, hydraulic connectivity, hydric soils, and human modifications such as levees. This project developed a guidance document on how to delineate floodways for several streams of various types, primarily based on gradient and degree of confinement. Deliverables included GIS data layers delineating the SMA floodways for several streams of varying types. A report was generated describing GIS and field delineation steps, data used, and sources available, analysis of differences between the SMA and FEMA floodways (e.g. area extent) and what if any predictive relationships were discovered between stream types and floodway types.
To address this objective, the regulatory framework in Washington State governing the delineation and management of flood-prone lands including floodplains, floodways and frequently flooded areas was reviewed and summarized. In addition the current scientific literature regarding use of remotely sensed data (including LiDAR, color infra-red photography, multispectral satellite imagery and high resolution photography) for identification and delineation of floodprone areas was also reviewed and relevant references were cited. A list of data sources was compiled. Live links were created for direct data download or to the data steward when appropriate.
The general approach to identification and mapping involved: 1) breaking the stream into reaches based on natural features (e.g. points of confluence, change in gradient or confinement); 2) overlaying soils data that was spatially attributed with information on hydric, permeability, erodability information; 3) overlaying the Federal Emergency Management Act Floodplain (FEMA) maps on Light Detection and Ranging (LiDAR) coverage, soils, wetlands, and obstruction data (e.g. roads, railroads, levees, and culverts); and 4)selecting and highlighting the floodplain boundary segments that overlap contours that
- indicate sudden elevation drop into depression or tributary channel not included in mapped floodplain
- indicate erodable or floodprone soils not included in floodplain area
- indicate obstructions that are likely to affect floodplain in ways not indicated.
A decision tree was developed to systematically step through a review and visual analysis of each of these highlighted segments. These questions were used to refine confidence in the delineation and reduce quantity of highlighted segments. Rationale for removal of each segment from ‘questionable' status was documented. Further field assessment that would be useful in better assessing the reduced highlighted floodplain segments (e.g. vegetation types to identify, geomorphic information to gather) was also indicated.